In a multichannel telecommunication system using frequency-division-multiplex/frequency-modulation (FDM/FM) signaling, for example, one or more message channels can be ocassionally allocated to data transmission. For such mixed transmission with alternation between analog and digital signals, the digital signals could be relegated to a base band different from that of the analog signals of the selected channel or channels or could be elevated to the intermediate-frequency band, yet these two solutions involve inconvenience in circuit design. The simplest and therefore generally preferred solution is to transmit the digital signals or data in a low-frequency range replacing the base band of the analog signals.
In the latter type of telecommunication system it is known to transmit a pilot wave of predetermined frequency f.sub.p over such a channel along with the analog message signals in one mode of operation, that pilot wave being monitored by a sensor to test the continuity of the established connection; in another mode of operation, i.e. with transmission of digital signals, that pilot wave is usually suppressed. It is also general practice to equip the receiver with a detector for noise signals arising during transmission in a band of low frequencies not used for the analog signals, the detector responding to a central frequency f.sub.n of that band which is sometimes referred to as a noise window. In the second mode of operation, however, the range of frequencies required for high-speed data transmission generally encompasses the noise window, the pilot frequency f.sub.p and all or part of the frequency spectrum used in the first mode for the transmission of analog signals over the selected channel. Thus, different noise-frequency detectors and pilot-wave sensors would be required for the two modes of operation, especially if the channel capacity is to be fully utilized with high-speed data transmission requiring monitoring of the noise signals at the receiving end for proper evaluation of the incoming digital information.